Bone is a highly vascularized tissue. During development of the skeleton and regeneration of bone in adult animals, the process of angiogenesis is highly coordinated with the process of bone formation by osteoblasts. In endochondral bone formation an avascular cartilage model is laid down. Blood vessels grow into the hypertrophic cartilage, which then dies, followed by bone formation. During intramembranous bone development, osteoblasts differentiate from mesenchymal precursors, and lay down matrix and mineral, closely coordinated with capillary development. The mechanism by which the vascular response is coordinated with skeletal formation is unknown. The goal of this proposal is to elucidate the molecular and cellular mechanisms regulating capillary formation during bone growth and regeneration. We will determine the spatial and temporal distribution of key angiogenic factors in developing bones using morphologic approaches. This will be coupled with functional angiogenesis assays and perturbations of angiogenic activities in vivo and in culture. We will exploit mice with genetic mutations that affect skeletal development (e.g., gelatinase B/matrix metalloproteinase-9 null mice) to dissect specific steps in the process. We will then determine how angiogenesis is regulated during the programs of cartilage and bone differentiation using in vivo and in culture models. We will determine whether the angiogenic processes operating during skeletal development are recapitulated during bone regeneration after injury. We will explore the molecular and cellular mechanisms and mechanical forces that regulate angiogenesis during bone regeneration in normal mice, and in mice with genetic mutations that affect bone formation. Conditions that allow bone to repair by intramembranous ossification or endochondral ossification will be used so that the effects of different mechanical environments on angiogenesis can be evaluated. Because angiogenesis is a key process in bone formation, the capacity to develop strong, functional bone during fetal development and to repair bone and maintain skeletal integrity in adults may critically depend on it. Once the molecular signals required for regulating angiogenesis are understood, they will provide targets for new therapies for skeletal lesions occurring in development, bone regeneration and in osteoporosis.